It is now well established that cells cultured in 3D express quite different suites of genes compared to cells grown in 2D monolayers, and that they more closely mimic in vivo behaviors. Moreover, cell-cell and cell-matrix crosstalk also significantly affect cell responses to external stimuli. It is therefore reasonable to suppose that small molecule screens for cancer therapeutics using complex 3D tissues will be more likely to yield new and effective lead compounds than screens against single cell types grown in 2D. Here we propose a novel 3D tissue system, ideally suited for high throughput screening (HTS) of anti-cancer drugs, that is physiologically relevant, physically robust, and scalable. This "tissue-in-a-bead" system consists of a small vascular bed, stromal cells and tumor cells, grown in a physiologic extracellular matrix (ECM) environment and encapsulated by a tough, transparent, and hollow, polymeric bead. Each cell type will express a different fluorescent protein to allow remote, repeated sampling of the number and/or viability of each cell type in the tissue following exposure to different drugs. The beads will be approximately 1-3 mm in diameter and will be porous to macromolecules. The mechanical and physicochemical properties of the beads will allow for easy handling using robotic fluidic devices. Importantly, the system is scalable in that, regardless of the number of beads used - single beads for drug screens or multiple beads for when tissue recovery is desired - the local environment seen by the cells will not change and so re- optimization of culture conditions for a new geometry will be unnecessary. It is important to note that we are not proposing that these tissues will accurately model all the complexity of real tumors - a goal that is currently beyond reach - rather, we suggest that a model system that includes the basic components of a tumor, including endothelium, matrix, stromal cells and tumor cells in a 3D microenvironment, will provide a much better screening platform than cells cultured alone and/or in 2D. PUBLIC HEALTH RELEVANCE: We are proposing a system to improve the efficiency of drug discovery, especially of anti-cancer drugs. We will create miniature tumors inside beads that contain blood vessels and other support cells. These can then be handled without damaging the tissue, allowing robotic systems to screen millions of potential drugs against a tumor that closely mimics those that grow in the body. Ultimately this system may reduce the cost of drug discovery.